1. Field of the Invention
The present invention relates to a piezoelectric resonator and a piezoelectric resonance device for use as different kinds of resonators, oscillators, or similar apparatuses, and more particularly, to a thickness extensional piezoelectric resonator and a piezoelectric resonance device each operative to use higher harmonics of a thickness extensional vibration mode.
2. Description of the Related Art
Piezoelectric resonators are used in a variety of piezoelectric resonation devices such as piezoelectric oscillators, piezoelectric filters, and so forth.
Japanese Unexamined Patent Publication No. 1-117409 discloses an energy-trap type piezoelectric resonator operative to utilize the second harmonic in a thickness extensional vibration mode. This piezoelectric resonator will be described with reference to FIGS. 22 and 23.
The above-mentioned piezoelectric resonator is formed by laminating ceramic green sheets 61 and 62 made of a piezoelectric material, and firing them integrally, as shown in the exploded perspective view of FIG. 22. A circular excitation electrode 63 is provided on the ceramic green sheet 61 in the center thereof. The excitation electrode 63 is led out to one of the side edges of the ceramic green sheet 61 through a lead electrode 64. Further, a circular excitation electrode 65 is provided in the center of the upper side of the ceramic green sheet 62. The excitation electrode 65 is led out to one of the side edges of the ceramic green sheet 62 through a lead electrode 66. An excitation electrode 67 is provided on the underside of the ceramic green sheet 62, and is lead out to the side edge of the ceramic green sheet 62 through an lead electrode 68, as shown in the downward projection.
The above-mentioned ceramic green sheets 61 and 62 are laminated, pressed in the thickness direction, and baked. The obtained sintered material is polarized whereby a piezoelectric resonator 70 as shown in FIG. 23 is produced.
In the piezoelectric resonator 70, piezoelectric layers 71 and 72 are uniformly polarized in the thickness direction, namely, in the direction indicated by the arrows in FIG. 23.
The piezoelectric resonator 70 can be resonated by connecting in common the excitation electrodes 63 and 67, and applying an AC voltage between the excitation electrodes 63, 67, and 65. In this case, the vibration energy is trapped in the area where the excitation electrodes 63, 65, and 67 overlap, which defines a resonance portion A.
The prior art piezoelectric resonator 70 which is operative to use higher harmonics in a thickness extensional vibration mode is provided as an energy-trapping piezoelectric resonator, as described above. Accordingly, it is necessary to provide a vibration-attenuating portion at the periphery of the resonance portion A for attenuation of the vibration. That is, it is necessary to provide the vibration attenuating portion having an area which is larger than that of the resonance portion A. Therefore, it becomes difficult to miniaturize the piezoelectric resonator 70.
Japanese Unexamined Patent Publication No. 2-235422 discloses an energy-trap type piezoelectric resonator containing a strip-type piezoelectric ceramic, which does not require a piezoelectric substrate portion having a large area located at the periphery of its resonance portion.
In the energy-trap type piezoelectric resonator 80, an excitation electrode 82a is provided on the upper side of an elongated piezoelectric substrate 81, and an excitation electrode 82b on the underside thereof. Each of the excitation electrodes 82a and 82b is arranged to extend to a pair of the long sides of the piezoelectric substrate 81, that is, to extend over the entire width. Further, the back side of the excitation electrode 82a and the front side of the excitation electrode 82b are opposed to each other in the center in the longitudinal direction of the piezoelectric substrate 81 whereby the resonance portion is defined by the overlapped portions. Further, these excitation electrodes 82a and 82b are extended to the ends 81a and 81b in the longitudinal direction of the piezoelectric substrate 81.
In the piezoelectric resonator 80, unnecessary vibrations are generated during the excitation of the thickness extensional vibration mode. Japanese Unexamined Patent Publication No. 2-235422 describes that when a fundamental wave is utilized, the ratio of WIT (width/thickness) equal to about 5.33 at a resonance frequency of 16 MHz is preferred, and when the third harmonic is utilized, unnecessary spurious components between the resonance frequency and the anti-resonance frequency can be reduced by setting the ratio of W/T at about 2.87 when the resonance frequency is about 16 MHz. In other words, when higher harmonics in a thickness extensional vibration mode are practically utilized, various unnecessary spurious vibrations appear in addition to the spurious components between the resonance and anti-resonance frequencies. Accordingly, a problem occurs that effective resonance characteristics can not be obtained.
Also referring to the piezoelectric resonator disclosed in Japanese Unexamined Patent Publication No. 2-235422, the electric capacity is relatively small so that the piezoelectric resonator is susceptible to a floating capacity generated from the circuit board other components.